Originally Posted by Pearl2011
Its higher on its own, and get even higher with the sand.
So ticked off right now.
I would doubt the sand is doing this. I'll explain.
Tap water has some amount of dissolved CO2 in it; this varies, depending where you are. When testing tap water for pH, always either let the tap water sit overnight, or put some in a bottle and shake it very briskly for a couple minutes. This out-gasses the CO2. Then test the pH. Depending upon the amount of CO2, the pH will be higher after the CO2 is out-gassed than it was prior. CO2 creates carbonic acid that naturally lowers the pH.
Try this and see what you get for the tap water. Which then should be close to the tank water. Playsand has no calcareous substances in it to affect water chemistry, at least my playsand doesn't.
It is possible the municipal water can change in pH. The source of the water may be variable due to environmental factors from season to season; there may be more than one source and the water in each source may be different; or the municipality may add some substance to alter the pH. The latter is common if the source water is acidic, since this has a tendency to gradually dissolve water pipes, appliances, water heaters, etc because of the acidity, so they add sodium ash or whatever to keep the pH higher.